Poly(lactic acid) (PLA) is a widely used polymer derived from natural sources. PLA is a hydrophobic polymer that is completely insoluble in water. The hydrophobicity of PLA can be attenuated by copolymerization with other hydroxycarboxylic acids such as glycolic acid and 4-hydroxybutyric acid (Amy et al., (2004) J. Biomater. Sci. Polym. Edn. 15: 1281-1304; Lu et al., (2000) Biomater 21: 1837-1845; Biomaterials and Bioengineering Handbook, ed. W. DL, Marcel Dekker, New York, pp. 141-155). The tunable hydrophobicity has previously been exploited in drug delivery applications, but it is also desirable for the absorption of organic pollutants (Wang et al., (2010) J. Biomed. Mat. Res., Part B, Appl. Biomater. 93, 84-92; Liu et al., (2006) J. Biomed. Mat. Res., 78A: 798-807; Liu et al., (2005) Nanotechnology 16: S601-S608; Liu et al., (2006) Int. J. Nanomed. 1: 541-545). PLA has been considered suitable for these purposes because of its biocompatibility, tunable biodegradation and controlled hydrophobicity (Yanling et al., (2005) J Macromol Sci C Polym Rev. 45: 325-349; Hiltunen & Harkonen, (1997) Macromol. 30: 373-379), and make PLA ideal as an environmentally-acceptable sorbent material.
Titania (titanium dioxide, TiO2) is a biocompatible metal oxide commonly used for anti-fouling, anti-microbial, and UV-absorbing properties. Titania has well known photocatalytic properties. It can be used to degrade most organic chemicals to CO2 and water. The photocatalytic properties of titania have thus far only been observed in the anatase and rutile crystalline forms but not the amorphous phase (Zhang, (2009) Coord. Chem. Rev. 253: 315-3041; C. C. Sorrell, (2011) J. Mater. Sci. 46: 855-874). There have been many methods developed to form anatase phase or rutile phase titania. All of these methods require harsh conditions such as strong acids or bases or high temperatures which are not compatible with polymeric systems, precluding combining photocatalytically active TiO2 and heat-sensitive organic polymers in a single structure (Ismagilov et al., (2009) Rus. Chem. Rev. 78: 873-885; Wu et al., (2009) Eur. J. Inorg. Chem. 2009: 2789-2795; Xin, (2010) Appl. Mater. Inter. 2: 3479-3485; Kalita (2006) Mater. Sci. and Eng. A. 435-436: 327-332). Recently, however, methanol has been shown to induce the mineralization of titania into a photocatalytically active mixture of anatase and amorphous phases at low temperature and without the use of acids or bases.
There have been efforts to incorporate titania into polymer systems to utilize the desirable properties of both constituents. Such combination materials are multifunctional, being able to absorb and degrade organics, be biodegradable, and are biocompatible (environmentally benign). Because of these uses, PLA based composites have mostly been used as protective coatings.
There been few reports on PLA/titania systems prepared in situ under relatively mild conditions. All previous studies have been focused on incorporating pre-prepared titania into a polymer matrix. A variety of different approaches have been used to create mixed-hybrid PLA/titania systems such as mixed composites (Zhu et al., (2011) Polym Composite 32: 519-528), grafted to polymers (Luo et al., (2009) Acta Materials 57: 3182-3191), and by modifying TiO2 for dispersal in composite systems (Norio Nakayama, (2007) Polym Deg Stab 92: 1255-1264).
Most of the work on developing PLA/TiO2 composite systems has been for the purpose of bioengineering bone grafts. The PLA/TiO2 systems have better performance than the previously studied PLA/hydroxyapatite systems. The TiO2 reduces the acidity of the bone graft as the PLA degrades into lactic acid, and also increases the overall degradation rate. Thin films, microspheres and microfoams have been employed for this purpose. The only studies on PLA/TiO2's photocatalytic properties have focused on thin films for applications such as antifouling coatings. All of these systems were also composite systems and had issues related to inconsistent mixing, while most also exhibited a lag time associated with mass transfer limitations of hydroxyl radicals out of the PLA matrix. This lag time was eliminated by exposing the films to UV irradiation before exposing the films to a test dye solution.